Oncolytic adenovirus (Ad) vectors belong to a new class of cancer therapy agents that destroy cancer cells as part of the virus’s lytic infectious cycle. In this chapter we describe an immunocompetent, semi-permissive cotton rat tumor model to evaluate the safety and efficacy of oncolytic Ad vectors. With this model one can investigate the effect of the host immune system on the vector-tumor interaction as well as the vector’s effect on normal host cells in vivo. This chapter describes procedures for analyzing the growth and cytolytic properties of oncolytic Ad vectors in cotton rat cells in vitro. We discuss handling and husbandry issues and techniques for subcutaneous, intratumoral, and intravenous injection of cotton rats. We present methods for generating subcutaneous tumors in cotton rats and assessing the efficacy of Ad vectors upon intratumoral injection. Also, we discuss procedures for determining the biodistribution of a replicating Ad in cotton rats.